Impact of inductive current transformers on synchrophasor measurement in presence of modulations

Instrument Transformers (ITs) are essential devices for all the measurement tasks, f.i. metering, protection and diagnostics, in electricity grids, since they scale voltage and current down to levels compatible with measuring instruments. At the input of Power Quality (PQ) instruments and Phasor Measurement Units (PMUs), inductive voltage and current transformers (VTs and CTs) are still the most used ITs, even if, their performance in presence of disturbances is not fully assessed. This paper analyses the impact of the inductive CTs on the measurement of fundamental synchrophasors, by a PMU, in presence of a particular class of disturbances, the undesired modulations on current signals. Experimental tests, on two different CTs, show that there are critical situations, where, even compensating CT systematic ratio and phase errors, 1) CT exceeds its accuracy class limit and 2) considering the sole contribution of CT, PMU performance stays very close to the error limits of the standards.